It is well known that Brillouin Optical Time Domain Reflectometers (BOTDR) are sensitive to both the strain and temperature state of the optical fiber being interrogated. Historically, the main challenge to these systems has been to separate the strain effects from the temperature effects in order to measure both quantities. The invention described here provides a system and method for discriminating the contribution of the signal change due to both of these quantities.
Previously used methods have included techniques which measured not only the shift in the Brillouin frequency but also the bandwidth of the gain spectrum. Also, attempts have been made to strain isolate the cable using mechanical design as well as installing a separate fiber to perform a differential measurement.
Optical distributed temperature sensors, commonly referred to as “DTS” systems, based on fiber optic sensing techniques are being used broadly in a number of applications and markets (for more information, see www.sensa.org). Optical DTS is predominantly based on nonlinear type optical sensors, in which high intensity pulsed laser energy is launched into a sensing fiber to stimulate nonlinear effects that cause light scattering. Optical DTS systems have been made using optical Raman effects, and other optical DTS systems have been made using optical Brillouin effects.
It is known that both Raman effects and Brillouin effects cause both Stokes anti-Stokes shifted signals propagating in both forward and backward directions in which their relative intensity and/or frequency is dependent on temperature. Raman effects and Brillouin effects are discussed in the paper, Daniele Inaudi and Branko Glisic, “Integration of distributed strain and temperature sensors in composite coiled tubing”, 2006 SPIE Smart Structures and Materials Conference, San Diego, Calif., Feb. 27 to Mar. 2, 2006, (Authors from SMARTEC SA, Via Pobiette 11, CH-6928 Manno, Switzerland, www.smartec.ch), the disclosure of which is hereby incorporated by reference in its entirety. Using Optical Time Delay Reflectometry (OTDR), temperature at distinct positions all along the fiber can be derived so that the entire fiber is probed as a fully distributed temperature sensor.
Of the nonlinear DTS sensors, the use of Raman type far exceeds that of the Brillouin type because the Raman effect—being vibrational—is sensitive to temperature only, as compared to the Brillouin effect (acoustic) that is sensitive to both temperature and strain. Use of the latter for DTS therefore requires complete isolation of fiber strain or extraction of its strain error in the temperature measurement. Conversely, Brillouin systems are frequently used to monitor strain in known or controlled thermal environments.
Despite insensitivity to strain, there are drawbacks to Raman systems in that the intensity of backscattered Stokes/anti-Stokes signals are very weak, requiring a high sensitivity optical receiver and significant amount of signal averaging/processing because of the low received optical signal-to-noise ratio (OSNR). Furthermore, with the Raman shifted lines being widely separated in wavelength (e.g. over 200 nm for a 1550 nm operating system), the received intensity of these lines, and subsequent temperature measurement, can be significantly offset by changes in background fiber attenuation. This problem is commonly referred to as differential fiber attenuation (DFA) and is exacerbated when using telecommunications-grade graded index multimode fiber.
The above-mentioned multimode fiber is used predominantly in Raman systems because of its high level of index-modifying dopants such as GeO2 to increase the nonlinear scatter intensity as well as being better to collect scattered light. Unfortunately, this fiber is quite sensitive to environmental effects such as chemical (hydrogen) and mechanical events acting on the fiber that cause DFA that can be of significant magnitude in the case of the former and random in the case of the latter. Furthermore, such multimode fibers have an inherent higher attenuation rate compared to single mode fibers and therefore are not well suited for long reach (e.g. power lines, subsea oil and gas pipelines). Raman systems can and do operate on single mode fibers that can benefit from the lower sensitivity to DFA by virtue of their single mode waveguiding and lower dopant level; however the intensity and light collection of Raman scattered signals is quite low with single mode fibers, relegating single mode Raman systems primarily in applications where single mode fiber is already in place (e.g. optical ground wires) or short reach applications.
Brillouin systems, on the other hand, operate exclusively on single mode optical fibers that are less sensitive to DFA by their single mode propagation (no differential modal attenuation) and lower dopant level. Furthermore, Brillouin systems are further insensitive to residual DFA as the separation between lines is much smaller—only fractions of nanometers—so that changes in background fiber attenuation tends to apply almost equally on the two lines. Brillouin DTS systems thus become attractive in a range of long reach and chemical environments if not for the strain cross-sensitivity inherent with Brillouin technology, leading to a number of proposed methods to eliminate or isolate strain acting on the fiber through mechanical design as well as through the installation of a separate fiber to perform a differential measurement. Such methods also include means to discriminate or separate temperature-modulated information from corresponding strain-modulated information.
These methods have included techniques which measure not only the shift in the Brillouin frequency but also the bandwidth of the gain spectrum. Among these methods, reference is made to Parker et al. “Temperature and strain dependence of the power level and frequency of spontaneous Brillouin scattering in optical fibres”, Optics Letters, June 2007 Vol. 22, No. 11, pp. 787-789 and Parker et al. “Simultaneous Distributed Measurement of Strain and Temperature from Noise-initiated Brillouin Scattering in Optical Fibres”, IEEE JQE, April 1998, Vol. 34, No. 4, pp 645-659.
In addition to monitoring the amplitude and bandwidth of the frequency shifted signal, solutions using multi-layered fibers or dispersion shifted fibers which generate multiple peak return signals have also been investigated. See “Spontaneous and Stimulated Brillouin Scattering Gain Spectra in Optical Fibers”, Yeniay, Aydin et al. Journal of Lightwave Technology, VOL. 20, NO. 8, August 2002. Both of these approaches suffer from a multitude of practical issues including poor contrast of the various peak shifts relative to one another as well as poor sensitivity of the amplitude and bandwidth to differing strain and temperature. These practical issues have limited the realization of such systems.
In view of the foregoing, there is an ongoing need for a strain-temperature sensing system which is configured to provide independent information about the strain and temperature states of an optical fiber.